When does Cherenkov radiation occur?

Light slows down when travelling through water. Its speed is only about 75% of normal. So particles emitted from nuclear fuels are able to move faster than light in the watery medium. The charged particles interact with water molecules, releasing photons and creating a shockwave of blue or violet light.

Cherenkov radiation happens when electrically charged particles, such as protons or electrons, travel faster than light in a clear medium like water. When this happens, the water molecules and particles interact to give off light.

https://www.energy.gov/ne/articles/cherenkov-radiation-explained

Soviet physicist Pavel Cherenkov shared the 1958 Nobel Prize in Physics for the 1934 discovery of Cherenkov radiation.
https://en.wikipedia.org/wiki/Cherenkov_radiation